This invention relates generally to the processing of measured data. Especially the invention applies to the binning procedure of data, which is measured with a CCD (Charge-Coupled Device) sensor unit. The invention is preferably used in photo-metrics for measuring radiation from samples on a well plate. One purpose of the invention is to achieve improvement in signal-to-noise values of the measurements.
CCD sensors are generally used in photometrics for measuring radiation, such as light, from samples. The samples are usually inserted into wells on a well plate in photometries equipment. Next one implementation of such a measurement is explained as an example. However, this implementation is not meant in any way to restrict the field of use of the present invention.
FIG. 1 shows a prior art arrangement of a photometrics device 100. The samples to be measured are inserted onto a well plate 102. The samples may be excited with radiation from a lamp unit 104. The excitation radiation 106 is reflected by a beam-slitter mirror 108 onto the well plate. The lamp unit 104 is controlled to give radiation with a determined intensity. After a possible excitation, the radiation 110 from the samples is led to a CCD unit 120. A lens system 112 creates an image of the samples onto a CCD screen. The exposure period is controlled with a shutter 116. The radiation is further filtered with an emission filter 114 in order to select the determined wavelength of radiation for the measurement. The lamp unit 104, the shutter 116 and the CCD unit 120 are controlled with a control unit 130. The measurement process is further controlled by a computer unit 140. The computer unit also processes the output data of the measurements to achieve radiation intensity results.
FIG. 2 illustrates registers of a CCD unit 200. The CCD unit comprises a parallel register 210 that consists of a matrix of charge wells 211. As the surface is exposed to radiation, charges are formed into the charge wells according to the intensity of the radiation exposure. A charge pattern thus accumulates in the parallel register. After the exposure the charge wells or “pixels” of the matrix are read by shifting the charges at each row of the parallel register towards a shift register or “serial register” 220. After shifting the charges by one step, the charge wells of the shift register comprise charges of one column of the parallel register. The shift register is then read by shifting the charges at the shift register towards an output charge well or “output node” 230. After each step of shifting the output node is read. After all the charge wells of the shift register are read, the charges at the parallel register are further shifted by one step. The readout procedure is further repeated until the whole parallel register is read. The measurement data is thus converted into serial set of pixel charge values that present radiation intensities at the pixels. FIG. 2 also shows images of four samples 203 of a well plate. After processing the ouput data, an image can be formed where pixels within the sample image area present the radiation intensity of the corresponding positions within the sample image.
One problem in photometrics is related to the fact that the intensity of the radiation is low and therefore the signal-to-noise ratio of the measurement data is often low. In order to increase the signal-to-noise ratio, binning method is often used. Binning is a technique of combining charge from adjacent pixels during the readout process. The charge is collected as described above, but the readout is programmed differently. With parallel binning, when charge is shifted from the parallel register into the shift register, charge is accumulated from two or more columns before the serial shifting begins. With serial binning, two or more charge packets are similarly accumulated in the output node before the charge is digitalized and read out.
FIG. 3 illustrates how groups of pixels in a CCD unit are combined into larger “super pixels”. Binning is specified by a binning factor, which is the number of pixels to be combined on the CCD. For example, “8×8 binning” is used in FIG. 3, which means that each group of 8×8 is accumulated in the binning. Thus the CCD of 40×40 pixels is grouped into 5×5 super pixels in the readout (A1–A5, B1–B5, C1–C5, D1–D5 and E1–E5). If binning is used, the capacity of the shift register and the output charge well must be designed according to the total charge of the pixels that are accumulated in binning.
Binning improves the signal-to-noise ratio and extends the dynamic range of the CCD imager, but at the expense of spatial resolution. Binning is thus useful in applications where resolution is not of primary concern. Because binning reduces the number of pixels to be processed and digitized, the readout speed is also increased. If, for example, 2×2 binning is used, the resolution (number of pixels in the corresponding direction of the image) becomes half of the corresponding resolution without binning, and the signal-to-noise value becomes almost twice as good as the corresponding value without binning. This improvement of signal-to-noise value is related to averaging the noise from the parallel register of the CCD unit. Therefore, the signal-to-noise value related to the readout noise improves even by the binning factor. Binning is advantageous in photometrics, because there has been no need for obtaining high resolution.
One problem relating to CCD units is the fact that there are often defects in the charge wells, which serve as pixels. In general, it is very difficult to produce a CCD unit with no such defects. In economical mass production of CCD units it is usual that there is, just for example, one defected pixel in 1000 pixels in average. In most applications of CCD units it does not have a significant effect if a few pixels of a CCD unit are defected. However, if CCD unit is used in accurate measurements, and especially, if binning is used, then a few defected pixels may decrease the quality of the measurements significantly. In this patent application the denomination “defected” means that the charge well does not function according to a determined specification, which causes that its ability to convert radiation into charges or its ability to maintain the accumulated charge or its ability to transfer a charge from/to its neighbouring charge well is worse than what is required. “Defected” may also mean that additional charges are formed into the charge well thus causing a “white defect”.
In FIG. 3 there are shown two defected pixels 361 and 362. When binning is used in the readout the two defected pixels cause an error in measured value of the whole super pixels B2 and C4. And additionally, in the readout process there are also other pixels whose charges are shifted to the serial register through the defected pixels. These pixels 371 and 372 are marked with diagonal lines in FIG. 3. When the charge wells of the pixels 361 and 362 are defected, they may not retain the charges shifted through the defected charge wells in the correct value. Therefore the value of the charges from all the pixels 371 and 372 may be distorted when the charges are accumulated into the serial register. This causes an error in the measured value of the super pixels C2, D2, E2, D4, and E4 as well. As a result the measured values of 7 super pixels out of 25 are incorrect because of defects only in two pixels out of 1600. It is clear that the capacity of the measurement equipment is thus significantly degraded. On the other hand, if special CCD units with no defects would be manufactured, this would make the measurement equipment too expensive for many measurement applications.